kubeadm: add notes about deprecating kube-dns usage in 1.18 (#18851)

* kubeadm: add notes about deprecating kube-dns usage in 1.18

* implementation-details: update notes about DNS

content/en/docs/reference/setup-tools/kubeadm/implementation-details.md

@@ -447,21 +447,11 @@ A ServiceAccount for `kube-proxy` is created in the `kube-system` namespace; the
 
 #### DNS
 
-Note that:
+- In Kubernetes version 1.18 kube-dns usage with kubeadm is deprecated and will be removed in a future release
-
 - The CoreDNS service is named `kube-dns`. This is done to prevent any interruption
 in service when the user is switching the cluster DNS from kube-dns to CoreDNS or vice-versa
-- In Kubernetes version 1.10 and earlier, you must enable CoreDNS with `--feature-gates=CoreDNS=true`
+the `--config` method described [here](/docs/reference/setup-tools/kubeadm/kubeadm-init-phase/#cmd-phase-addon)
-- In Kubernetes version 1.11 and 1.12, CoreDNS is the default DNS server and you must
+- A ServiceAccount for CoreDNS/kube-dns is created in the `kube-system` namespace.
-invoke kubeadm with `--feature-gates=CoreDNS=false` to install kube-dns instead
-- In Kubernetes version 1.13 and later, the `CoreDNS` feature gate is no longer available and kube-dns can be installed using the `--config` method described [here](/docs/reference/setup-tools/kubeadm/kubeadm-init-phase/#cmd-phase-addon)
-
-
-A ServiceAccount for CoreDNS/kube-dns is created in the `kube-system` namespace.
-
-Deploy the `kube-dns` Deployment and Service:
-
-- It's the upstream CoreDNS deployment relatively unmodified
 - The `kube-dns` ServiceAccount is bound to the privileges in the `system:kube-dns` ClusterRole
 
 ## kubeadm join phases internal design

content/en/docs/reference/setup-tools/kubeadm/kubeadm-init-phase.md

@@ -157,6 +157,8 @@ dns:
   type: "kube-dns"
 ```
 
+Please note that kube-dns usage with kubeadm is deprecated as of v1.18 and will be removed in a future release.
+
 For more details on each field in the `v1beta2` configuration you can navigate to our
 [API reference pages.] (https://godoc.org/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2)
 

content/en/docs/reference/setup-tools/kubeadm/kubeadm-init.md

@@ -67,10 +67,14 @@ following steps:
 
 1. Installs a DNS server (CoreDNS) and the kube-proxy addon components via the API server.
    In Kubernetes version 1.11 and later CoreDNS is the default DNS server.
-   To install kube-dns instead of CoreDNS, the DNS addon has to be configured in the kubeadm `ClusterConfiguration`. For more information about the configuration see the section
+   To install kube-dns instead of CoreDNS, the DNS addon has to be configured in the kubeadm `ClusterConfiguration`.
-   `Using kubeadm init with a configuration file` below.
+   For more information about the configuration see the section `Using kubeadm init with a configuration file` below.
    Please note that although the DNS server is deployed, it will not be scheduled until CNI is installed.
 
+   {{< warning >}}
+   kube-dns usage with kubeadm is deprecated as of v1.18 and will be removed in a future release.
+   {{< /warning >}}
+
 ### Using init phases with kubeadm {#init-phases}
 
 Kubeadm allows you to create a control-plane node in phases using the `kubeadm init phase` command.

content/en/docs/tasks/administer-cluster/coredns.md

@@ -63,6 +63,10 @@ In Kubernetes 1.11, CoreDNS has graduated to General Availability (GA)
 and is installed by default.
 {{< /note >}}
 
+{{< warning >}}
+In Kubernetes 1.18, kube-dns usage with kubeadm has been deprecated and will be removed in a future version.
+{{< /warning >}}
+
 To install kube-dns on versions prior to 1.13, set the `CoreDNS` feature gate
 value to `false`: